In highly dispersive pseudo-linear high bit-rate long-haul transmission systems, such as submarine or transcontinental terrestrial light wave transmission systems, intra-channel four-wave mixing (IFWM) and intra-channel cross-phase modulation (IXPM) are the main causes for nonlinear penalties. Use of a differential-phase-shift-keying (DPSK) modulation format can mitigate these penalties as compared to on-off-keying (OOK). It has also been shown that the reduced pulse energy of Return-to-Zero (RZ)-DPSK compared to RZ-OOK and a correlation between the nonlinear phase shifts of two adjacent pulses contribute to increased robustness of RZ-DPSK against IFWM.
In principle, alternating the polarization of the signal from bit to bit in two orthogonally polarized signals will further reduce the nonlinear penalties since there is little FWM between orthogonal polarization states. An improvement in the transmission distance can hence be achieved by using alternate polarization-DPSK (APol-DPSK) compared to single polarization DPSK. The simultaneous application of DPSK and polarization alternation enhances the performance in an optical transmission system where the intra-channel optical nonlinear transmission system where the intra-channel optical nonlinear interactions are a limiting factor.
With increasing data-rates, in particular up to or even more than 40 Gbit/s, polarization mode dispersion (PMD) becomes the physical effect that limits the transmission distance of an optical fiber.
Even worse, PMD is a statistical effect due to the environmental dependence of birefringence and mode-coupling of a single-mode fiber. This means that, with some probability, the instantaneous differential group delay (DGD) can be much higher or lower than the mean DGD, i.e. PMD, of the fiber.
It is to be emphasized here that when a signal experiences unacceptable high distortion due to accumulated PMD over the desired transmission distance, an active and adaptively adjustable compensation method is required to countermeasure this type of signal degradation.
Known feedback controlled optical PMD compensators (PMDC) for compensating for fiber PMD are not applicable for modulation formats using orthogonal polarization components, i.e. APol or orthogonal multiplexing formats.
Therefore, a need exists in this field of endeavor to provide a method and an apparatus for compensating polarization mode dispersion introduced by an optical transmission fiber when transmitting optical signals having a first polarization component and a orthogonal second polarization component.